1. Field of the Invention
This invention relates to babesiosis vaccines and their preparation.
2. Description of the Prior Art
Babesiosis is a disease which most domestic animals may experience, and is caused by various species of the tick-borne protozoan parasite Babesia. In the cattle industry in particular, there are many parts of the world where the disease poses serious economic problems. For instance, in Australia during the year 1972 cattle losses directly attributable to the tick-babesiosis disease complex were estimated to be of the order of $42 million.
The only effective vaccines against babesiosis which are currently available involve the use of living organisms, and the production of infection to induce acquired immunity. To avoid the risks inherent in the use of living organisms, vaccines which would confer immunity without infecting the host have been sought for many years. Some success has been obtained by immunization with antigen from the blood of infected animals, but concurrent immunization of the recipients with the blood group antigen has precluded the widescale use of products containing such impurities. The invention described herein concerns a method of preparation of babesiosis vaccine derived from non-living antigenic material but which avoids the shortcomings of earlier art non-living vaccines.
In events leading up to the development of the present invention it was found initially that cattle inoculated with crude suspensions of dead Babesia bovis parasites prepared from infected blood were protected against babesiosis even after challenge with strains of virulent organisms. These preparations were unsuitable for practical vaccination because they were contaminated with bovine erythrocytic material, and were likely to cause haemolytic anaemia in the calves of vaccinated cows due to the production of incompatible blood-group antibodies by the dam and their transfer to the offspring.
However, subsequently to this discovery further efforts were directed towards the purification and characterization of the babesial antigens contained by infected red cells that confer protection against babesiosis on uninfected cattle. A number of different babesial antigens were identified in the infected red cell and on the surface of the parasite itself. The first was a soluble antigen present in the red cell cytoplasm and released with the haemoglobin when the infected cell was lysed. As the protective antigens obviously remained in the red cellparasite residue after lysis, this antigen was of no further interest. Sonic disintegration of the insoluble residue after lysis of infected red cells produced a solution that contained two groups of antigens. One group consisted of aggregates of fibrinogen-like protein which contained at least 2 antigenic molecules. One of these antigens was located in a dense band around the rim of the infected erythrocyte and the other was distributed in fine granules on the erythrocyte stroma. The complex was designated the Fibrinogen Associated Antigen (FAA) and was purified from solution by precipitation techniques that were suitable for the isolation of fibrinogen. Thus it was found that one group of antigens consisted of babesial proteins conjugated to fibrinogen molecules to form large aggregates that behaved chemically like fibrinogen and could be purified as such. However, it has now been realised that, surprisingly, another antigen remained in the solution after precipitation and was located on the surface of the parasite. It was not associated with any red cell structure. The fact that this soluble fraction contained an antigen was only discovered after in vivo testing in cattle wherein antibodies were produced after injection of antigen into the host. This in vivo testing was carried out as a final check after the antigen could be not detected by normal serological tests such as haemagglutination. This new antigen is termed Soluble Parasitic Antigen (SPA). It has been found that both FAA and SPA induce protection against Babesia bovis infection even though the antibodies produced to each are of different specificity. However, FAA contains the bovine blood group antigens but SPA does not. For this reason SPA is of interest in development as a vaccine for babesiosis.